If you already have an electronic tag, it will work on Westlink M7, so you're all set.

Visitor's e-PASS

The Roam Visitor's e-PASS is designed for visitors to Sydney who need cover for all Sydney toll roads for up to 30 days. Apply for a Visitor's e-PASS at www.roam.com.au or by calling 13 TOLL (13 8655).

The symbol is used on road signs in Sydney to identify toll roads that accept passes such as e-PASS.